The present invention relates to a planar antenna that is small in size and low profile, and is adapted to operate in a plurality of frequency bands.
As a conventional planar antenna having a small size and low profile, an M-type antenna having a flat radiating electrode is disclosed in Japanese Patent Publication No. 5-136625A, which will be described with reference to FIGS. 10 and 11.
In the conventional M-type antenna as shown in FIG. 10, a radiating electrode 12, which is formed of a flat conductive plate and whose planar outer shape is square, is disposed to be spaced apart from a grounding plate 10 and parallel to the grounding plate 10. A feeding pin 14 is erected from the side of the grounding plate 10 and is electrically connected to an approximate center portion of the radiating electrode 12. In addition, at approximately symmetrical locations relative to the location where the feeding pin 14 is disposed, a pair of short-circuiting pins 16 are provided such that center locations of outer edge portions of two opposing sides of the radiating electrode 12 are electrically connected to the grounding plate 10. The feeding pin 14 is electrically isolated from the grounding plate 10.
The VSWR characteristics of the M-type antenna having such a structure is illustrated in FIG. 11. As an example, it is adapted to operate in a single frequency band around 900 MHz.
Further, as an antenna adapted to operate in two frequency bands, the technology combining two M-type antennas each formed of a conductive wire is disclosed in Japanese Patent Publication No. 2002-359515A.
Referring to FIGS. 12 to 14, a brief explanation will be given of the M-type antenna disclosed in this publication. In the structure shown in FIG. 12, a first radiating electrode 18 formed of a conductive wire is arranged away from and in parallel to a grounding plate 10, and at a nearly central position thereof, a feeding pin 14 is caused to be erected from the side of the grounding plate 10 and electrically connected to the first radiating electrode 18. A pair of short-circuiting pins 20 are provided so as to electrically connect both ends of the first radiating electrode 18 to the grounding plate 10. At the intermediate position of the feeding pin 14, a second radiating electrode 22 formed of a conductive wire is arranged in parallel to the grounding plate 10 and the first radiating electrode 18. The feeding pin 14 is connected to the nearly central position of the second radiating electrode 22. A pair of short-circuiting pins 24 are provided so as to electrically connect both ends of the second radiating electrode 22 to the grounding plate 10. The horizontal directivity characteristics of the M-type antenna having such a structure is illustrated in FIGS. 13 and 14. As seen, it is not considered that both first and second radiating electrodes 18, 22 has omni-directivity on the horizontal plane.
Meanwhile, recent electronic devices having functions to support various media and services require an antenna capable of operating in a plurality of frequency bands. In addition, generally, the installing space for the antenna is limited. The M-type antenna disclosed in Japanese Patent Publication No. 5-136625A can operate only in a single frequency band. So, in order to communicate a plurality of frequency bands, another antenna must be additionally mounted. In this case, such another to be added is obliged to be aside or above the radiating electrode 12. This correspondingly requires a wide or tall installing space.
The recent electronic devices having functions to support various media and services are mostly mounted on a vehicle and are required to have omni-directivity on the horizontal plane of the antenna. The M-type antenna disclosed in Japanese Patent Publication No. 2002-359515A can operate in two frequency bands while making the installing space smaller, However, there is a drawback in that its horizontal directivity is not omni-directive.